Gamma prime (.gamma.') precipitation strengthened nickel-base superalloys are widely used in gas turbine engines because they exhibit a desirable balance of creep, tensile and fatigue crack growth properties at elevated temperatures. .gamma.' precipitation strengthened nickel-base superalloys are distinguishable from other nickel-base superalloys not only by their .gamma.' phase, but also by the applications for which they are particularly suited. For example, the .gamma." precipitation strengthened nickel-base superalloys taught by U.S. Pat. No. 5,143,563 to Krueger et al., assigned to the same assignee of the present invention, are adapted to form polycrystalline articles such as turbine disks, in which a particular grain size distribution is necessary in order to achieve required mechanical properties at elevated temperatures.
Such superalloys derive desirable properties from the presence of precipitates and alloying constituents at the grain boundaries of the alloy. As an example, boron and carbon form borides and carbides at the grain boundaries of such nickel-base superalloys, which advantageously serve to promote crack growth resistance and time dependent properties, and are therefore typical alloying constituents for superalloys used to form turbine disks. Notably, boron is required in turbine disks in order to achieve adequate dwell fatigue crack growth resistance and creep resistance at elevated temperatures.
In contrast, nickel-base superalloys such as those taught by U.S. Pat. No. 4,719,080 to Duhl et al. and U.S. patent application Ser. No. 08/270,528 to Wukusick et al., the latter being assigned to the same assignee of this invention, are directed to single crystal articles, such as turbine blades. Because such articles are intended to lack grain boundaries, precipitates and alloying constituents which have a beneficial effect when present at the grain boundary are generally unnecessary and possibly undesirable. For example, nickel-base superalloys used to form single crystal articles often intentionally exclude carbon and boron as constituents.
To achieve optimal properties in .gamma.' precipitation strengthened nickel-base superalloys, components such as turbine disks are typically formed by powder metallurgy methods which entail a consolidation step, such as extrusion consolidation. The resulting billet is then isothermally forged at temperatures slightly below the alloy's .gamma.' solvus temperature to approach superplastic forming conditions, and thereby promote filling of the die cavity. These processing steps are designed to retain a fine grain size within the material, avoid fracture during forging, and maintain relatively low forging loads.
In order to improve the fatigue crack growth resistance and mechanical properties of the resultant forged article at elevated temperatures, the article undergoes a heat treatment above the superalloy's .gamma.' solvus temperature (generally referred to as supersolvus heat treatment), during which significant, uniform coarsening of the grains occurs. At such high heat treatment temperatures, the .gamma.' phase is dissolved but then later reprecipitated upon quenching of the forged article.
As the material requirements for gas turbine engines have increased, various processing methods have been suggested to enhance the mechanical properties of the components. For example, components such as turbine disks have been processed to have coarser grains on the order of about ASTM 9 and coarser, particularly at the rim of the disk, in order to enhance their high temperature properties. (Reference throughout to ASTM grain sizes is in accordance with the standard scale established by the American Society for Testing and Materials.) To maximize the mechanical properties of such components, grain sizes within the component must be generally uniform, preferably limited to a range of several ASTM units.
In addition, compositions for .gamma.' precipitation strengthened nickel-base superalloys have also been tailored to optimize properties at elevated temperatures. For example, advanced high strength nickel-base superalloys typically have been alloyed to attain high volume fractions of the .gamma." phase, on the order of 40 volume percent and more, necessitating a higher heat treatment temperature to dissolve the .gamma." phase.
The fine nickel-base superalloy powders required to produce components having optimal properties are typically prepared using an argon-atomizing process, which generally involves melting ingots of a superalloy in an argon gas atmosphere, and then atomizing the liquid metal using argon gas. While argon-atomizing methods have distinct advantages over other powder production techniques, the billet produced by consolidation of the powder may contain entrapped gaseous argon.
The entrapped argon later expands during the high temperature supersolvus heat treatment to form gas bubbles or pores in the forged article, an undesirable condition termed thermally induced porosity (TIP). These pores are often associated with grain boundaries, depending on their mechanism of formation. The pores significantly reduce the low cycle fatigue properties of the forged article by serving as preferential sites for crack initiation. For reasons not entirely understood, certain .gamma.' precipitation strengthened nickel-base superalloys are particularly vulnerable to thermally induced porosity.
In the past, porosity in numerous types of alloys has been reduced by employing hot isostatic pressing (HIP) techniques. HIP processes serve to eliminate internal voids and microporosity through a combination of plastic deformation, creep and diffusion, the result of which produces a denser article. However, hot isostatic pressing complicates the processing of the article, adds undesirable costs to processing, and may not always sufficiently reduce porosity for more demanding applications.
Accordingly, it would be desirable if a method were available by which the tendency for thermally induced porosity could be significantly reduced. In particular, such a method would retain the desirable argon-atomizing process by which a fine superalloy powder is formed, yet would reduce the tendency for argon entrapped in the superalloy to expand during supersolvus heat treatment. In addition, such a method would be compatible with the production of nickel-base superalloy articles such as turbine disks, in which a uniform coarse grain microstructure is necessary to achieve desirable mechanical properties at elevated temperatures.